Conventional multi-user communication systems use frame-based (or packet-based) transmission to communicate between two or more users over a shared channel based on OFDM. Examples of such systems include IEEE 802.11x (Wireless LAN), IEEE 802.16 (WiMAX) and ITU G.9960 (G.hn). These systems use OFDM transmission (also referred to sometimes as Discrete MultiTone (DMT)) which divides the transmission frequency band into multiple sub-carriers (also referred to as tones or sub-channels), with each sub-carrier individually modulating a bit or a collection of bits.
Conventional methods of packet detection use a predefined preamble as specified, for example, in the standards above. The G.hn standard uses predefined OFDM symbols modulated with an all ones bit as specified in Section 7.1.4.5.2.1.1 of G.hn and rotated with the Constellation Scrambler in Section 7.1.4.3.3 of G.hn. For convenience, the text contained in Sections 7.1.4.5.2.1.1 and 7.1.4.3.3 of G.hn is contained in Appendix A. A method for supporting multiple reference sequences to modulate the preamble is ITU Contribution 09BM-038 (contained in Appendix B). 09 MB-038 proposes to allow different reference sequences to modulate the preamble subcarriers. The different sequences may be defined by a properly chosen set of seeds that initialize the already specified LFSR generator. This LFSR (Linear Feedback Shift Register) generator provides the pseudo-random di-bit sequence that modulates the non-masked preamble sub-carriers. Allowing the use of different reference sequences among different networks can serve as another mechanism for mitigating disturbances from neighboring networks in PLC environments.